Apparatus and method for the operation of cells for the igneous electrolysis of alumina



J y 1969 c. BOULANGER ETAL 3, 5 ,7

APPARATUS AND METHOD FOR THE OPERATION OF CELLS FOR THE IGNEOUS ELECTROLYSIS OF ALUMINA Filed Jan. 6, 1965 2 Sheets-Sheet 1 Flg.1

IN VENTORJ CLAUDE 5 OULA/VGEQ JE/P/Wf PAIL/5315A? -fi/1 0/V July Filed 15, 1969 c. BOULANGER ETAL APPARATUS AND METHOD FOR THE OPERATION OF CELLS FOR THE Jan. 1965 IGNEOUS ELECTROLYSIS 0F ALUMINA 2 Sheets-Sheet 2 z L 0 T t -'as -50 4s 40 -}s -50 -ns 4b 435 A51 t4 *6! k1 \55 -54 ng.z'

INVENTORS CLAUDE BOUL A/VGEA L $506M; Paummp-Tmvwv United States Patent US. Cl. 204-67 8 Claims ABSTRACT OF THE DISCLOSURE A method for operating cells employed for igneous electrolysis of alumina wherein the internal ohmic existence of the cell is measured at intervals for purposes of detecting a pattern of increasing resistance which indicates the onsetting of scorching. Alumina is supplied to the cell when such increases are detected whereby scorching is avoided.

The present invention relates to a new method of anticipating scorching of and systematically feeding cells for the igneous electrolysis of alumina. The invention also relates to an apparatus for achieving the new method.

It is well known that cells for igneous electrolysis employed for the preparation of aluminum must be so supplied with alumina that the alumina content of the electrolytic bath remains between two limits. The lower limit is defined as the limit below which polarization of the anode system, called scorching, is likely to occur. The upper limit is that limit above which there is observed a formation, at the bottom of the cell, of deposits of undissolved alumina which are likely to contaminate the carbon of the cathode plate, thereby seriously interfering with the operation of the cell.

It is one object of this invention to provide a method for operation in conjunction with cells provided for the igneous electrolysis of aluminum whereby anticipation of scorching and systematic feeding of the cells with alumina can be accomplished.

It is a further object of this invention to achieve an automatic procedure of the type referred to in the foregoing object which insures at all times the proper alumina content in the electrolytic bath and which provides for anticipation and suppression of conditions which would cause scorching.

It is an additional object of this invention to provide an apparatus suitable for carrying out the foregoing objects.

These and other objects of this invention will appear hereinafter and for purposes of illustration, but not of limitation, a specific embodiment of the invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a circuit diagram illustrating the elec: trical system providing for the regulation of alumina feeding and providing for the anticipation of scorching; and,

FIGURE 2 is a graphic illustration demonstrating the procedure employed for determining the approach of scorching conditions.

In the method of the invention, the cell is supplied with alumina whenever the value of the internal ohmic resistance of the cell commences a regular increase, which is a sign of scorching. In a preferred embodiment of this method, certain specific operations are performed. First, the approximate internal resistance, called the observed 3,455,795 Patented July 15, 1969 resistance R, of the cell is measured by calculating the ratio between the ohmic voltage drop U in the cell and the current I which passes through the latter. U is determined by the difference between the voltage V existing across the terminals of the cell and an estimation a of the counterelectromotive force of the cell.

Thereafter, the mean value of R is calculated, this value being taken over a period greater than one second but less than 10 minutes, so as to obtain a mean value [R] which does not depend upon the fluctuations of the momentary internal resistance R. The position of the anode system is then subjected to the value of [R] and, finally, the cell is supplied with alumina when the value of [R] commences a regular increase which is a sign of scorching.

The apparatus of this invention comprises a phase comparator having a saturable magnetic circuit made up of four windings. The first winding is connected through a resistor to the terminals of a shunt through which the current I of the cell flows. The second winding forms part of a circuit comprising, in series, a first rheostat, a unidirectional-voltage source whose voltage a represents the counterelectromotive force of the cell, and the cell to be tested, the cell and the source being connected in opposition. The third winding is connected to an alternating-voltage source of frequency f (c./s.), and the fourth winding supplies a harmonic detector which provides a voltage of frequency 1 phase shifted by r2 or ,u2 in relation to the voltage of the alternating-voltage sources.

The apparatus also includes a biphase motor having one of its circuits connected to the alternating-voltage source and the other through an amplifier to the harmonic detector. The shaft of the motor acts upon the first rheostat, and there are also provided an indicator for reading R and a second rheostat.

In addition, the apparatus comprises a mean-value circuit including, in series, the second rheostat, a directcurrent generator, an integrating circuit, an ammeter displaying the value of [R] and a comparison circuit which compares [R] with a reference value R and which acts upon the raising mechanism of the anode circuit of the cell.

In a modern electrolysis workshop, the large dimensions of the room, the sudden variations of current due to scorching and the impracticality of having personnel constantly available make the adjustment of the position of the anode system of the cells a tedious and inaccurate op eration. The idea of replacing routine manual adjustment by automatic regulation thus naturally springs to mind. However, prior attempts to achieve such regulation have not provided the desired results.

In order to provide the desired automatic adjustment, it is necessary in the first place that regular operation of the cells should be possible and that it should not be modified by secondary operations. This does not present any great difliculty in the case of cells comprising prebaked anodes. In the case of cells comprising continuous anodes of the Soederberg type, it is necessary to arrange for the removal of the tars while introducing only small disturbances into the adjustment of the cell. These tars must be regularly removed in accordance with a fully prepared plan.

Objective information concerning the adjustment of the plane limiting the lower end of the anode system (the anode plane) is given by the value of the real internal resistance p of the cell. This resistance is defined by:

in which a is an approximate and constant estimation of E.

FIGURE 1 illustrates three cells of the series under consideration, i.e., 1, 2 and 3, the apparatus being connected to the cell 2. The calibrated shunt resistor 11, which is in series with all the cells, supplies one of the exciter windings 13 of a magnetic comparator 12 through a resistor 16. This winding is therefore traversed by a current of strength i proportional to I, which current passes through the cell: i=AI. The second exciter winding 14 of the comparator 12 is supplied through a first rheostat 19 of resistance r with a voltage obtained by providing a source 18 of voltage a in opposition to the potential difference V existing across the terminals of the cell 2. This winding 14 is therefore traversed by a current equal to:

The saturable magnetic circuit 17 of the comparator 12 comprising a third winding 15 supplied by an alternating-voltage source 21 of any frequency f, which is here equal to 400 c./s. A fourth winding 16 supplies the harmonic detector 20. The latter selects the harmonic 2 of the alternating current supplied by the generator 21 and then converts this harmonic current into a sinusoidal current of frequency which is phase-shifted by in relation to the current supplied by 21.

The biphase motor 23 has its fixed phase winding 24 connected to the terminals of the generator 21, while its control-phase winding 25 is connected through the amplifier 22 to the output of the detector 20.

The motor 23 drives the slider of the first rheostat 19 as well as a direct reading indicator 26 optionally provided with a device for recording the momentary internal resistance; and the slider of a second rheostat 27.

The rheostat 27 forms part of a circuit through which there flows a current I proportional to the mean value [R] of R. The circuit comprises, in series, a directcurrent source 28, an integrating device 29 and a comparator device 30 which compares the value [R] with a refer ence value R and sends through the time limit device 31 a regulating order to the motor for shifting the anode system of the cell 2. The current I actuates a measuring device 32 graduated in [R], and the measuring device may be provided with a recording device.

Terminals 41 to 44 are provided so that the device may be connected to any one of the cells.

In operation, the currents i=AI and ampere-turns number, a unidirectional magnetic field saturates the magnetic circuit 17 of the transformer formed by the windings 15 and 16 in combination with the magnetic circuit. This sets up in the winding 16 a deformed alternating current comprising, in addition to the fundamental frequency, the even harmonics. This deformed current acts upon the detector 20, which selects the harmonic 2 of frequency 2 and then converts it into a current of frequency f phase-shifted in relation to the current flowing through the winding 15 by depending upon the sign of the unidirectional magnetic field which saturates the magnetic circuit 17 and upon which the polarity of the harmonic 2 depends. This signal passes through the amplifier 22 and acts upon the winding 25 of the motor 23, of which the other Winding 24 is directly supplied by the generator 21. The direction of rotation of this motor depends upon the sign of the phase shift of the current supplied by the detector 20' and therefore upon the sign of the unidirectional magnetic field in the circuit 17, which in turn depends upon the relative magnitude of i and 1'. When i=j, the field is zero, the detector 20 fed with an undeformed voltage of frequency 1 no longer feeds the winding 25, and the motor 23 stops. At this instant, the following applies:

i=j, that is to say:

and hence V a I At equilibrium, the observed momentary resistance R of the cell 2 is therefore proportional to r, which is the resistance of the rheostat 19. Now, when equilibrium does not exist, the motor rotates and acts upon the rheostat 19 until equilibrium is established. The displacement of the motor is therefore proportional to R, of which the value is displayed and optionally recorded by 26.

It has been observed that the internal resistance of a cell undergoes oscillations of considerable amplitude. It was also found that there exist permanent oscillations whose period is in the neighborhood of one second and whose amplitude is in the neighborhood of one microhm, these oscillations being connected with the electroylsis phenomenon itself and probably being due to the evolution of gas by which it is accompanied. In addition, it has been found that there exist slower fluctuations whose period is in the neighborhood of one minute and whose amplitude may reach five microhms. The latter fluctuations, which seem to be connected with the movements of the metal, which in turn result from an unbalance in the distribution of the current, are larger in proportion as the electrolytic cell itself is more powerful, and they appear to be smaller in cells having prebaked anodes than in cells having Soederberg anodes.

The rapid variations of the internal resistance constitute one of the great difliculties which have been responsible for the failures encountered in the application of automatic regulation. It is therefore desirable to eliminate or lessen these variations by basing the regulation, not on the momentary value R of the resistance, but on a mean value [R] taken over a time longer than one second and shorter than 10 minutes, but generally between 15 seconds and two minutes.

For this purpose, the motor 23 drives a second rehostat 27 which is supplied by a generator 28 supplying constant unidirectional voltage. The voltage is supplied through an integrator 29 which may simply consist of a resistance-capacitance system, or preferably of an equilibrium circuit of the Wheatstone bridge type, of which the time constant is substantially equal to the time over which it is desired to take the average. This integration may, however, be carried out by any other mechanical or electrical device.

With the described arrangement, there flows through the circuit of the generator 28 a current strength I pro portional to [R]. In the circuit 30, the value of J is compared with the value I which corresponds to the reference value R of R and the current thus obtained can control, through a relay, the motor of the device for raising the anode system of the cell 2.

It is prefer-able to compare the value of I by means of relays included in 30 and having different fixed values J J J which constitute the limits of successive zones, and it is also preferable to give a regulating order which is a function of the zone in which [R] has been detected. In addition, it is preferred to bring about the regulation only at fixed intervals of time, The time limit device 31 performs this function. FIGURE 2 illustrates an embodiment of such a regulation.

The time t before the scorching is plot-ted along the abscissae and the variations MR] of the observed internal resistance [R] as read or recorded at 32 are plotted along the ordinates. The circuit 30 distinguishes, for the variations MR] the following zones:

The zone 0, reference 51, between the values of MR] corresponding to the ordinates 5 6 and 58;

The zone +1, reference 52, between the values 56 and 5 7 of MR];

The zone 1, reference 54, between the values 5 8 and 59 of MR];

The zone +2, reference 53, corresponding to the values of MR] above 57;

The zone 2, reference 55, corresponding to the values of MR] below 59.

The circuit 30 gives in accordance with the position of MR] the following orders:

No order if the value of MR] is in the zone 0;

A number of revolutions, n, in the rising direction (+11 revolutions) if the value of MR] is in the zone -1;

A number of revolutions, p, in the rising direction if the value of MR] is in the zone +2, p naturally being higher than n;

A number of revolutions r in the descending direction (r revolutions) if the value of MR] is in the zone +1;

Ignition of a supply signal which may simply be a luminous signal without adjustment if MR] is in the zone +2.

The circuit 30 permits variations of the zone widths up to 0.5 o, that is to say 0.05 volt for 100,000 amperes, and the values of p, N and r. A mean-adjustment button is also provided for varying the reference value R that is to say, for shifting all the zones for each of the cells being regulated. The regulation takes place only at fixed intervals of time assumed to be equal to 30 minutes in FIGURE 2, in which the times at which the regulation can occur are represented by the arrows 61 to 64.

The unbroken curve 65 shows how MR] changes before scorching, in the absence of adjustment of the interpolar distance or manual adjustment, while the broken line curve 66 shows the same curve with the regulator in operation. It will be seen that at the instant 105 minutes corresponding to the arrow 61 the value of MR] is in the zone 0. Therefore, the circuit 30 does not send any automatic regulation signal. On the other hand, at the instant -75 minutes, arrow 62, the value of MR] is in the zone +1, and the regulator acts and restores the value of MR] to the zone 0. The same action takes place at the instant +45 minutes, arrow 63. On the other hand, at the instant-15 minutes corresponding to the arrow 64, the curve 66 is in the zone +2 and no adjustment order is given, but a luminous signal is ignited, which constitutes an order for supplying alumina to the cell, as will be explained.

During the operation of the cell, disregarding for the instant the phenomena connected with the variation of the dissolved alumina content of the bath, two opposed phenomena take effect: the layer of fused aluminum rises, while the anode is consumed. These two phenomena have similar speeds, so that, in order to conform to the interpolar distance, it is sufficient to set with moderation in the upward or downward direction on the anode system. It is not possible to determine a permanent value for this fundamental adjustment, but the adjusting device described produces this action absolutely correctly.

Certain work also disturbs the adjustment of the anode. For example, disturbances are recognized due to the raising of the movable frame supporting the. anode system and, in the case of Soederberg anode cells, due to the removal of the tars which modifies the internal resistance. This modification must be kept so small as not to bring about starting of the regulation, by performing the removal of the tars of one anode in stages. The supplying of alumina, which produces a modification of the interpolar distance, resulting in a raising of the movable frame owing to the regulation, and the tapping of the aluminum, which may necessitate interruption of the regulation and manual restoration of the desired value of the internal resistance, the regulation being too slow, are also disturbance factors. The latter disadvantage may be eliminated by introduction of a regulation zone 3 situated below the zone 2 described and producing a high number of revolutions, above p, in the rising direction.

Finally, the observed internal resistance, that is to say that which is measured by the described device, depends upon the dissolved alumina content of the electrolytic bath. Since this content continually falls between two successive operations for the supply of alumina to the cell, a slow and continuous rise of this resistance is observed, whether it be a question of R or of [R]. This rise is also corrected by the automatic device. When the dissolved alumina content falls below a value such that the instant at which the polarization of the anode, or scorching occurs, i.e., the zero instant of FIGURE 2 is close, an accelerated rate of rise is observed. This rise, of which the duration is from two to three hours, does not at first exceed the regulation zone 52 and it is corrected by the regulator, but it constantly accelerates to the point where it represents more than one zone between two actions of the regulator, as occurs at the tim 45 minutes, corresponding to the arrow 64. The regulator is then placed out of circuit and the cell is supplied 1 with alumina at the appealance of the aforesaid luminous signal.

After this supply, a sudden fall of the observed internal resistance is observed, followed by a gradual fall of parabolic form, with stabilization at the end of to minutes. Although applicants do not wish to be limited by any particular theory, it is thought that two phenomena are involved here. First, the mass of alumina and crusts (the crust is chipped off before supplying) falls to the bottom of the cell, thus causing the level of the metal to rise, which results in a reduction of the interpolar distance and thus in a sudden fall of the observed resistance. Secondly, the gradual dissolution of the alumina in the bath changes the characteristics of the electrolyte.

It will be understood that it is unnecessary to formulate any theory as to the true cause of the variation of the observed internal resistance, which may be eithel a variation of the electrolyzing counterelectromotive force or a variation of the real internal resistance. The function of the regulator, which results in the provision of information as to the instant when it is desirable to supply the cell with alumina, is of cajor importance.

The alumina content of the bath must constantly remain within two limits. The lower limit is the level which is slightly higher than the content which produces scorching, and the upper limit corresponds to the point of saturation of the electrolytic bath with dissolved alumina. It is very important that this limit should not be exceeded, because the alumina, which has fallen to the bottom of the cell, would remain there and contaminate the cathode. This would seriously interfere with the operation of the cell, resulting in a gradual decrease of the output and ultimately completely preventing electrolysis from occurring. The quantity of alumina supplied at each sup ply operation is empirically determined so as to remain distinctly below the limit content corresponding to saturation of the bath.

In an important variant of the described method, the zone +2 is omitted, so as to allow the regulator to act in all circumstances as soon as the curve 66 rises above the ordinate 56. The supply of alumina is controlled by the number of actions of the regulator in zone +1. Experience shows that in the case of Soederberg cells of 100 kiloamperes, this number of actions is between 1 and 10, and generally between 1 and 5. In the case where 6 actions are exceeded, the anticipation time is short and the supply must take place almost immediately, scorching often taking place before there has been time to effect the supply. This number of actions is greatly influenced by the relative speeds at which the anode consumption and the rise of the metal take place. In practice, with the cells under consideration, the number of actions chosen is three or four when it is desired to have available a fairly long period, for example 2 hours, before effecting the supply, which makes it possible to combine the operations for a certain number of cells, and five or even six actions are desirable when it is preferred to supply fairly quickly, for example in a period of the order of half an hour after signalling. One of the factors determining the choice of the number of actions is the volume of the bath, the other factors remaining unchanged. In cells having a very small bath volume and in the case of cells having prebaked anodes, the supply may be necessary at the very first action of the regulator, while in the case of cells having a very large bath volume, the supply may take place only after a number of actions of up to 10.

To illustrate the operation of this invention, the method is applied to a series of 100-kiloampere cells having Soederberg anodes wherein the regulation is effected with the aid of the apparatus described. The extent of the anode displacement is evaluated at a number of revolutions of the shaft by which the displacements are effected, each revolution being equivalent to a displacement of the anode through one-third of a millimeter, i.e., to a voltage variation of 0.01 volt or to an internal resistance variation of 0.1 microhm. It may also be evaluated by the duration of the pulse imparted to the motor by which the anode system is displaced.

The regulator is successively connected to the cells of the series under consideration, each cell being adjusted every 30 minutes. For each cell, the measuring time is one minute 15 seconds in order to permit integration over a sufficient period of time, the regulation proper lasts 10 seconds, and the change-over to the succeeding cell by the operation of the contactors 42, 43 and 45 lasts seconds. The apparatus is therefore stopped for 1 minute 30 seconds per cell. An apparatus may thus serve 20 cells.

In this example, a value of 1.6 volts is chosen for w. The orders transmitted by the regulator are the following.

Zone -22 2:10 revolutions in the rising direction; Zone 1: n=2 revolutions in the rising direction; Zone 0: no order;

Zone +1: r=2 revolutions in the descending direction; Zone +2: ignition of the luminous supply signal.

In a first stage, a single cell was regulated, and over a period of four months the mean Faraday output of the regulated cell was 89.0 percent against 87.7 percent in the case of the manually adjusted reference cells, which were as similar as possible.

Thereafter, the regulation was applied to a group of five cells under the same conditions, but the zone +2 was omitted, the luminous supply signal being actuated by a storage relay when five actions in zone +1 had been observed after one supply.

The mean Faraday yield, taken over four months, was found to be equal to 88.0 percent, against 84.5 percent in the case of a series of unregulated cells which were as similar as possible. The yields were lower than those of the first example, because the cells concerned did not have the same thermal insulation and were operated under different conditions. The consumption of fluorine per ton of aluminum produced was 18 kg. against 30 kg. in the case of the unregulated reference cells.

A complete series of 20 cells was maintained in operation for several months with an adjustment every 30 minutes, so that a single measuring and regulating device is suflicient to ensure regulation of the cells of the series. The operation is very satisfactory and a reduction of the electrical consumption and above all of the fluorine consumption has been observed.

The described device is subject to many variations. In particular, the integrating device may be simplified if there is used for the measurement of theinternal resistance a device possessing a higher time constant and, perhaps, lower sensitivity. A much simpler damping device is then sufficient.

On the other hand, the resistance variations, and more particularly the slow variations, are smaller in the case of cells having prebaked anodes and the described integrating device is then unnecessarily complicated. In this case, it is suflicient to provide in the main circuit comprising the winding 16 of the comparator and the motor 23', an electrical or mechanical time constant, which permits omission of the integrating circuit 27 to 29. It is then the motor 23 which, through a comparator performing the function of 30, controls the motor which effects the displacement of the anode system of the cell.

It will be understood that various changes and modifications may be made in the method and apparatus of this invention without departing from the spirit thereof as described in the following claims.

That which is claimed is: 1. An apparatus for operation of a cell employed for the igneous electrolysis of alumina, said apparatus being utilized for measuring the internal ohmic resistance of the cell for detecting a regular pattern of increasing resistance which is an indication of scorching, said measuring being accomplished by measuring an approximate internal resistance R of the cell by calculating the ratio between the ohmic voltage drop U in the cell and the current I flowing through the latter, wherein U comprises the difference between the voltage V existing across the terminals of the cell and the estimated counterelectromotive force of the cell; calculating a mean value of R, which is taken over a period longer than one second but less than 10 minutes, so as to obtain a mean value [R] which does not depend upon the fluctuations of the momentary internal resistance R; and supplying the cell with alumina when the value of [R] commences a regular pattern of increase which is a sign of scorching, said apparatus comprising a phase comparator whose satura'ble magnetic circuit comprises four windings of which the first is connected through a resistor to the terminals of a shunt traversed by the current I of the cell;

the second forms part of a circuit comprising, in series, a first rheostat, a unidirectional-voltage source of which the voltage represents the counterelectromotive force of the cell, and the cell to be measured, the cell and the source being connected in opposition;

the third is connected to :an alternating-voltage source of frequency f in c./s.;

and the fourth supplies a harmonic detector which provides a voltage of frequency f phase-shifted by in relation to the voltage of the alternating-voltage source;

a biphase motor of which one of the circuits is connected to the alternating-voltage source and the other through an amplifier to the harmonic detector, the shaft of the motor acting upon the first rheostat, an indicator for reading R and a second rheostat, a mean-value circuit comprising, in series, the second rheostat, a direct-current generator, a circuit introducing a response time, an integrator, an ammeter displaying the value of [R], and a comparison circuit which compares [R] with a reference value R and which acts upon the mechanism for raising the anode circuit of the cell.

2. An apparatus in accordance with claim 1 in whic the circuit comprising the second winding of the phase comparator and the regulating circuit comprises contactors by means of which it can be successively connected to various cells supplied in series from a common current source.

3. An apparatus in accordance with claim 1 wherein the circuit provided for introducing a response time includes a damping device.

4. In the operation of a cell employed for the igneous electrolysis of alumina, the method comprising the steps of measuring the internal ohmic resistance of the cell for detecting a regular pattern of increasing resistance which is an indication of scorching, said measuring being accomplished by measuring an approximate internal resistance R of the cell by calculating the ratio between the ohmic voltage drop U in the cell and the current I flowing through the latter, wherein U comprises the dilference between the voltage V existing across the terminals of the cell and the estimated counter-electromotive force of the cell; calculating a mean value of R, which is taken over a period longer than one second but less than minutes, so as to obtain a mean value [R] which does not depend upon the fluctuations of the momentary internal resistance R; providing comparator means operating to subtract from the mean value [R] of the observed internal resistance of the cell a reference value R said Comparator means thus determining when the value A[R]=[R]R is, in absolute value, higher than a predetermined value, and operating means for supplying the cell with alumina when the value of A[R] commences a regular pattern of increase which is a sign of scorching, and wherein the range of values which may be assumed by A[R] is subdivided into zones, and wherein the operation depends upon the zone within which the observed value of [R] lies.

5. A method in accordance with claim 4, wherein the range of values which may be assumed by MR] is subdivided into five zones, and wherein the following orders for adjustment are applicable to the respective zones:

a zone 0, centered on the zero value of MR], to which there corresponds no order, the anode system not having to be displaced;

a zone +1 located between the upper limit of the zone 0 and a higher positive value, to which there corresponds a descending order applied to the mechanism actuating the anode system;

a zone 1 comprised between the lower limit of the zone 0 and a lower negative value, to which there corresponds a rising order applied to the mechanism actuating the anode system;

a zone +2 situated above the upper limit of the zone +1, to which there corresponds the despatch of an alumina supply signal, the anode system not being displaced;

a zone --2 situated below the lower limit of the zone -1, to which there corresponds a rising order applied to the mechanism actuating the anode system, the rising rate exceeding that of zone -1.

6. A method in accordance with claim 4 wherein the range of values which may be assumed by MR] is subdivided into four zones and wherein the following orders for adjustment are applicable to the respective zones:

a Zone 0 centered on the zero value of A[R], to which there corresponds no order, the anode system not having to be displaced;

a zone +1 situated above the upper limit of the zone 0, to Which there corresponds a descending order applied to the mechanism actuating the anode systerm;

a zone -1 located between the lower limit of the zone 0 and a lower negative value, to which there corresponds a rising order applied to the mechanism actuating the anode system;

a zone 2 situated below the lower limit of the zone 1, to which there corresponds a rising order applied to the mechanism actuating the anode system;

the alumina supply signal being sent when the order corresponding to the zone +1 has been given between one and 10 times.

7. A method in accordance with claim 6 wherein the alumina supply signal is sent when the order corresponding to zone +1 is sent between one and five times.

8. A method in accordance with claim 4 where the position of the anode system is subjected to adjustment depending on the value of A[R].

References Cited UNITED STATES PATENTS 2,545,413 3/1951 Perret-Bit 204225 2,918,421 12/1959 Lundborg 204-225 2,933,440 4/1960 Greenfield 204-67 3,317,413 5/1967 Chambran 204-245 XR 3,329,592 7/ 1967 Uhrenholdt 204-243 XR JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204-225-245; 318-18 

